Musicians are noise-exposed workers who heavily rely on their auditory sense and should protect it by wearing hearing protectors. However, many musicians do not protect their hearing because they find that the use of hearing protectors is detrimental to their performance, as they cause perceptual discomfort.
Professional musicians are exposed to high levels of sound and should protect their hearing to avoid permanent hearing loss that could compromise their career. Since high sound pressure levels are often required by a musician's work, the logical solution would be to wear hearing protection devices (HPDs) when required. However, perceptual discomfort associated with wearing HPDs can discourage musicians from using them. This perceptual discomfort is caused by two detrimental effects: the occlusion effect (OE) and the isolation effect (IE).
The occlusion effect (OE) is often reported as an unnatural and annoying perception of one's own voice or, instrument coupled to the skill, when wearing HPDs.
The isolation effects regroups acoustical and psychoacoustical factors that causes hearing protection users to experience perception shifts that an ideal hearing protection should not cause, making them feel isolated from their sound environment.
The isolation and occlusion effects are highly unfavorable to the musicians' auditory perception and compromise their capacity to perform to the best of their abilities for their audience. The isolation effect can make it difficult for musicians to judge the sound quality that is being presented to their audience. When, as a consequence of the occlusion effect, an augmented and unnatural perception of one's own voice or instrument is predominantly what is heard, musicians cannot hear the subtle cues that they depend on to adjust their playing. Cues such as knowing how their timbre blends with their colleague's or how loudly their instrument sounds and resonates in a given space can potentially make a big difference in one's performance. These adverse effects are causing some musicians to decide not to wear HPDs.
Oticon A/S′™ method for counteracting the occlusion effects described in U.S. Pat. No. 7,477,754 dated Jan. 13, 2009 is based on a generic fixed feedback controller with limited performance in terms of active control gain, because it does not account for wide inter-user variability, and therefore must make compromises regarding performance in order to be stable on all users, in various conditions. While such fixed feedback controller may be sufficient for hearing aid applications, where the external audio feed is played back at a sufficiently high level that partially masks the residual occlusion effect to reduce discomfort, it is in practice insufficient for other in-ear devices such as musicians earplugs or communication devices where a re-establishment of open-ear conditions would be desired.
Phonak AG™'s system and method for providing active hearing protection to a user described in US Patent Publication No:2011/0274284 A1 published on Nov. 10, 2011 features frequency band adjustments to correct the audio response of an in-ear device based on equal loudness across frequencies. The gain adjustments are limited in terms of frequency resolution, since they are limited to specific audio bands, and the gain factors are a function of a target loudness reduction that is selected by the wearer. With no means of assessing the true attenuation that is experienced by the user, which can vary greatly across users, these corrections cannot completely solve the isolation effect. These two limiting factors certainty affect the quality and usefulness of such device in hearing protection applications.
The occlusion effect (OE) is often reported as an unnatural and annoying perception of one's own voice when wearing HPDs. It will affect all musicians whose instrument induces vibrations to the skull, including singers and musicians whose instrument is pressed against any part of the head, such as a trumpet or violin. Although there is a direct solid borne sound path to the cochlea, it is generally accepted that the main objective occlusion effect is due to the existence of another solid borne sound path that ultimately reaches the cochlea by sound generation due to the vibrations of the ear canal walls that cause pressure fluctuations in the air contained in the ear canal. When the ear canal is unoccluded, less energy is transferred to the ear canal by bone conduction as the ear canal has an open-end, hence a lower acoustic impedance, and what is heard is predominantly the sound wave arriving from the air conduction path between the source (e.g. vocal tract) and the ear. However, when the ear canal is occluded, the walls have a strong coupling with the cavity and thus the ear canal sound level is greater and is picked up by the auditory system while the air conduction path is blocked, so what is heard is predominantly the sound wave traveling by bone conduction. Since this effect is more pronounced at low frequencies, below 1000 Hz, the result is an augmented and unnaturally “boomy” perception of one's own voice. FIG. 1 illustrates how the occlusion effect occurs.
It is apparent from FIG. 2 that the sound pressure level (SPL) increase caused by occlusion effect occurs in the lower frequencies of the speech bandwidth. The SPL in the occluded ear canal when one is speaking can typically amounts to 90 to 100 dB(SPL), and the occlusion effect results in an amplification of the low frequencies of the talker's own voice by up to 20 to 30 dB.
Occluding the ear with a HPD has an inherent effect on a wearer's auditory perception on multiple levels. The isolation effect (IE) regroups phenomena that cause a perception shift and/or a feeling of being isolated from a given sound environment. It originates from many causes: the different acoustical behavior of an open vs an occluded ear, the attenuation provided by typical HPDs, the impact of loudness perception on the perceived attenuation, and the fixed attenuation of most hearing protectors, sometimes resulting in over-attenuation.
The open, or unoccluded ear canal exhibits a wide acoustic resonance around 2.7 kHz, although the center frequency varies greatly among individuals. This resonance helps understanding speech consonants, which are mostly in this frequency region, and is a natural part of the way that we hear. Unfortunately, occluding the ear canal changes its acoustic properties and shifts the resonance to higher frequencies, to around 5.5 Hz and 8 kHz, as it is dependent on the remaining volume of the ear canal and its specific attributes. The shifting of this important resonance has two consequences, the first one is that a loss in sensitivity is felt where the natural ear resonance is supposed to occur, even without considering the attenuation of the earplug. The second consequence is that an increase in sensitivity is felt where the resonance of the occluded ear canal now is. Both consequences cause an unnatural perception shift.
Occluding the ear with a HPD typically results in an unbalanced attenuation, much more pronounced in the high frequencies than in the low frequencies. One reason for the low attenuation at low frequencies is that the earplug is free to vibrate because of its own flexibility as well as the flexibility of the ear canal flesh. This phenomenon is not significant at high frequencies, and a more pronounced attenuation is often seen, accentuated by the fact that occluding the ear shifts its natural resonance. Very small leaks between the device and the ear canal can also cause lower low frequency attenuation.
FIG. 3 shows typical shapes of non-uniform attenuation provided by an earplug-type HPD, an earmuff-type HPD, and both devices worn together, as well as the maximum attenuation limit of HPDs. This limit is imposed by the fact that sounds can bypass the HPD by bone and tissue conduction to the inner ear.
Loudness is defined as the perceived magnitude of a sound. it is a psychophysical magnitude strongly correlated to the physical magnitude of sound pressure level: one does not directly feel sound pressure level, one feels a loudness sensation caused by sound pressure level. Since loudness is frequency and SPL dependent, but in a non-linear way, a uniform decrease in SPL at all frequencies composing a sound does not usually translate to a uniform decrease in loudness at all frequencies of the sound. According to loudness models, if one was to wear perfectly uniform attenuation earplugs and another was hearing naturally, in the same given sound environment, they could feel different spectral balances: the relative difference in loudness between the frequency components would not be the same. This is analogous to what happens to perceived spectral balance as the volume of an audio material is turned up or down. In contrast, if a given earplug was not necessarily uniform in dB of attenuation, but was capable of producing uniformly decreasing loudness over the audio bandwidth, wearing or removing them would not have any effect on the perceived spectral balance. Since spectral balance assessment is used by musicians to blend their instruments together, adjust their playing, and even assess timbre, it is possible that the non-linearity of loudness perception is detrimental to the acceptance of uniform attenuation HPDs.
Accordingly, there is a need for an improved active hearing protection device and method therefore.